1. Technical Field
This invention relates to liquid level sensors, and more particularly to a digital liquid level sensing apparatus incorporating a dielectric constant differentiator for detecting variations in the dielectric of segmented portions of a capacitive probe.
2. Discussion
Liquid level sensors are used in a variety of applications to sense fluid levels in reservoirs where it is important or desirable to periodically or continuously monitor the level of the fluid within a reservoir. One form of liquid level sensor employs a capacitive probe having a pair of continuous, elongated elements (i.e., plates) positioned on a substrate of the probe. This form of sensing system makes use of the difference in the dielectric of air from various liquids. In such systems, some means is provided for generating a signal which is applied to one plate of the probe. The overall capacitance of the capacitor formed by the two plates, and thus the magnitude of the signal coupled onto the other one of the plates on the probe, will change as the percentage of the probe submerged in a fluid, and thus the two plates thereof, changes. Thus, the magnitude of the signal coupled onto the output plate of the probe can provide a relative indication of the area of the probe which is submerged in fluid and/or exposed in air.
Many prior developed systems incorporating capacitive probe technology have involved going to great lengths to fully characterize the dielectric constant of the substance whose level is being monitored in an effort to effect an accurate measurement of the level of the substance within a given reservoir. In some instances, such approaches have involved making some form of insitu measurement of the dielectric constant. Other approaches attempt to avoid the affects of the varying dielectric constant by attempting to remove the variation from the measurement. This is highly desirable because the dielectric constant of a given substance may vary to a significant degree when the substance experiences severe temperature changes or contamination from other substances which enter the reservoir. Thus, the overall accuracy of many such liquid level sensing systems incorporating capacitive probe technology can be greatly adversely affected by changes in the dielectric constant of the substance being measured as the composition of the subject is subjected to various environmental factors (e.g., temperature) and as the composition of the substance varies over a period of time.
One application where liquid level sensors are particularly desirable is with automotive vehicles. Recently there has been increasing interest in monitoring an even greater number of different fluids associated with motor vehicles to ensure that such fluids remain at optimum levels. For example, there has been increasing interest in incorporating sensing apparatus for sensing engine coolant levels, transmission fluid levels and differential case fluid, Just to name a few. The use of liquid level sensing apparatus with such fluids, however, presents a number of problems due to the extreme environmental changes which such a sensing apparatus must be able to tolerate, as well as the cost constraints which must be met in order for the apparatus to be economically mass produced without adding significantly to the overall price of the vehicle.
Recently released requirements, typical of the auto industry at large, for a fuel level sensor are listed below to provide an idea of the stringency of present day operational parameters which a fuel level sensor suitable for use in automotive applications must meet:
______________________________________ A. Temperature Range - (-)40.degree. C. to 150.degree. C. B. Life - 20 Years C. Response Time - preferably in the area of about or reasonably close to 15 milliseconds D. Accuracy 0.5 gallons minimum 0.1 gallons preferred E. EMI/RFI Must be operational in close proximity to fuel pump F. Fuel Tolerance - Sensor must be capable of meeting accuracy requirements for the following fuel types: TF1 TF2 UNLEADED GASOLINES 100% INDOLENE HO-III PEROXIDE FUEL MIX METHANOL FUEL MIX CORROSIVE GASOHOL Additionally, the sensor must be capable of limited exposure to 2 RVP Fuel as well as not being adversely affected by exposure to legal and commercial fuels in the Asian, Mideast and European markets. G. Underbody Contaminants - The sensor must withstand prolonged exposure to the following list of potential underbody contaminants: Engine Oil Transmission Fluid Power Steering Fluid Coolant/Antifreeze Brake Fluid Windshield Wash Fluid Transaxle/Differential Lube Wheel Bearing Lube Water A/C Refrigerant Snow, Ice Acid Rain Car Wash Chemicals Waxes, Paint Sealants Steam Cleaning Tire Cleaners Engine Cleaning Carpet Cleaners Soft Drinks, Coffee, Etc. H. Space/Size Requirements - The sensor shall be contained preferably reasonably close to the following form factors: 15 .times. 4 .times. 175 MM 15 .times. 4 .times. 400 MM 10 .times. 6 .times. 175 MM 10 .times. 6 .times. 400 MM I. Electrical Requirements - Operational Voltage: 10.5 to 16.5 volts Output voltage: 0 to 4.8 volts linearly related to measured level. J. Mechanical Requirements - Sensor must survive a three feet vertical drop and still meet the electrical requirements. ______________________________________
As mentioned above, to be suitable for use in automotive applications any liquid level sensor must meet the above requirements in addition to being capable of manufacture at a relatively low cost. This places an additional constraint on the design of the liquid level sensing system. In summary then the fluid level sensing system must accurately measure a variety of materials (i.e., fluids) in a hostile environment as well as being capable of economical manufacture.
Accordingly, it is a principal object of the present invention to provide a liquid level sensing apparatus incorporating a capacitive probe which senses the level of a liquid within a reservoir within which the capacitive probe is placed and which provides a sufficiently high level of accuracy which is not affected by changes in the dielectric constant of the substance being monitored.
It is another object of the present invention to provide a liquid level sensing apparatus which detects the level of a liquid within a fluid reservoir by detecting significant changes in the capacitance of a capacitive sensing probe having a plurality of segmented capacitors formed longitudinally thereon along an axis of measurement of the probe.
It is still another object of the present invention to provide a liquid level sensing apparatus capable of differentiating the dielectric constant of a substance at a plurality of points along a segmented capacitive probe disposed in the substance to thereby provide the capability of determining not only the point at which the capacitive probe becomes disposed in air, but also changes in the dielectric constant of the substance.
It is still another object of the present invention to provide a liquid level sensing apparatus which is economical to manufacture and suitable for use in hostile environments such as those encountered in various fluid reservoirs on a motor vehicle, and which meets or exceeds industry operating requirements.